Angular measurements are required in a wide variety of aerospace platforms such as airplanes, missiles, satellites etc. Typically, measurement of the angular rotation rates of such aerospace platforms is carried out by Attitude Heading Reference Systems (AHRS) or Inertial Navigation Systems (INS) that utilize three orthogonal rotation rate sensors (e.g. rate gyros) which are adapted for measuring the rotation rates of the aerospace platform with respect to three rotation axes. Typically, three orthogonal rotation rate sensors are aligned along the roll, pitch and yaw rotation axes of the platform and are thus adapted for sensing the rotation rate of the platform with respect to these axes.
Generally, having a priori knowledge of the platform's orientation, the orientation of the platform during its movement may be monitored by integration of the platform's rotation rates measured by respective rotation rate sensors (gyros) along three rotation axes of the platform. However, the accuracy of systems that determine the platform's orientation based on the initial platform orientation, and on continuous sensing and integration of the platform's kinematics depends on the accuracy of the priory knowledge of the initial state and is generally deteriorating during the platform's motion. This is due to errors of which the existing rotation rate sensors suffer (called bias drift effect) that impair their measurement accuracy. In particular, when using integration of measured rotation rates over extensive periods of time to determine changes in the platform's angular position, the bias drift may accumulate to substantial bias measurement errors which require repeated bias corrections.
Various techniques for compensating or reducing bias measurement error have been suggested. One method is to minimize the amount of bias corrections required by utilizing highly accurate rate gyros exploiting technologies such as ring laser gyros or fiber optic gyros. These types of gyros typically have small bias drift rates which enable to obtain accurate orientation readings (i.e. by integrating the rotation rate measurements) for substantial periods of time without requiring any external bias correction. However, implementing an AHRS or INS with these types of rate gyros is extremely costly.
Less-costly rotation rate sensors (such as MEMS gyros), which have high bias drift rates, require frequent bias corrections in the form of information from other sources relating to aerospace platform orientation. AHRS or INS systems utilizing these types of lower accuracy rate sensors are typically hybrid systems combining inertial measurements that provide continuous data relating to the orientation of the platform (i.e. based on an integration of the measured platform's rotation rates taken from aboard the platform) with data received from other sources (on-board or remote) providing, occasionally, additional accurate data relating to the platform's orientation about the pitch and yaw axes. The actual platform orientation is typically determined by utilizing statistical filtering algorithms, such as the Kalman Filter, to combine continuous data measured by the on-board rotation rate sensors and occasional data received from the additional sources. The additional accurate data relating to the platforms orientation enables to determine the bias drift of the on-board pitch and yaw rotation rate sensors and/or to correct the bias error accumulated during integration of the rotation rate measurements. Such techniques are limited at least because they provide correction only to the measurement of the pitch and yaw rotation rates measurement, while not providing correction to the measured roll rotation rate.
For example, some existing AHRS and INS systems are integrated with external (e.g. remote from the platform) measurement facilities, such as GPS-based positioning systems, which are adapted to measure and provide to the platform, occasional data indicative of the platform's orientation and/or location. However, relying on remote systems, such as GPS-based systems, does not provide sufficient integrity for driving systems with catastrophic failure modes, such as an AHRS.
According to other known techniques, inertial rotation rate measurement systems are integrated with on-board direct measurement systems, such as global positioning systems (GPS), that enable to accurately determine, from time to time, the platform's orientation. For example, rate gyro integrations are used to compute a platform's orientation while parameters obtained from other sensors or from direct measurement of the platform's orientation are compared, from time to time, with the computed platform orientation to determine proper bias values of the three rate gyroscopes. This allows compensation of measurement errors that are introduced by the bias drifts of the sensors. It should be noted that in many cases, direct measurement to systems provide better results when they are utilized during low dynamic motion of the platform. Accordingly, in some cases, during high dynamic movement (maneuvering) of the aerospace platform, inertial rotation rate measurement systems as used to determine the orientation of the platform, while when the platform is in lower dynamic motion, the results from direct measurement systems are used to compensate bias drifts of the inertial rotation rate sensors (gyros).
Direct measurements of the orientation of the platform with respect to the pitch and yaw axes are relatively straightforward (e.g. for the pitch rate gyro, using measurements of the earth's magnetic flux provides a reliable and accurate method for determining bias of the pitch and yaw gyros). However, determining the bias of the roll rate gyro bias is not nearly so straightforward.
Many systems rely on a human operator for the provision of roll angle corrections during platform movement. The main disadvantages of this method are the need of the foreknowledge of the initial/intermediate roll angle which is often based on human judgment, which makes such systems cumbersome and susceptible to human error.
Other techniques for determining the roll angle of an aerospace platform are based on the measurements of the earth's magnetic field. An example of such a method is disclosed in U.S. Pat. No. 4,608,641. This patent describes an aircraft operating in a gravitation field and having conventional sensors for measuring true air speed, angles of incidence and yaw, rotation about x, y and z axes, and acceleration therealong is provided with means for calculating the inertial component of the acceleration from data concerning the true air speed, heading and rotation of the aircraft obtained from the sensors, and means for comparing the inertial component with the total acceleration sensed, thereby to deduce the orientation of the gravitation component and hence obtain an estimate of the pitch and bank angles of the aircraft.
The above techniques, however, generally require accurate positioning information and determination of variations in the earth's magnetic field. Also, they have relatively poor accuracy in the vicinity of the earth's magnetic poles.
U.S. Pat. No. 4,608,641 discloses an aircraft operating in a gravitational field and having conventional sensors for measuring true air speed (sensors 10, 11), angles of incidence and yaw (sensors 13, 12), rotation about x, y and z axes and acceleration therealong (sensors 14, 15, 16) is provided with means for calculating the inertial component of the acceleration from data concerning the true air speed, heading and rotation of the aircraft obtained from the sensors, and means for comparing the inertial component with the total acceleration sensed, thereby to deduce the orientation of the gravitational component and hence obtain an estimate of the pitch and bank angles of the aircraft.
U.S. Pat. No. 5,886,257 discloses three rate gyros mounted to a ballistic body to provide an autonomous navigation system. A roll gyro, a yaw gyro and a pitch gyro are rigidly fixed to the ballistic body. Each gyro is arranged to be responsive to a roll rate about an input axis that is substantially orthogonal to any other gyro. The roll-rate gyro has its input axis aligned parallel to the body spin axis. An on-board processor utilizes recursive Kalman-filtering to determine the roll angle, i.e., the local vertical direction, from the gyro outputs.